Hydrogen (H) defect interactions have been investigated by molecular
statics simulations in tungsten (W), including H-H interactions and
interactions between H and W self-interstitial atoms. The interactions
between H and small H-vacancy clusters are also demonstrated; the
binding energies of an H, a vacancy and a self-interstitial W to an
H-vacancy cluster depend on the H-to-vacancy ratio. We conclude that H
bubble formation needs a high concentration of H in W for the H bubble
nucleation and growth, which are also governed by the H-to-vacancy ratio
of the cluster. The vacancy first combines with H atoms and a cluster
forms, then the H-vacancy cluster goes through the whole process of
vacancy capture, H capture, and vacancy capture again, and as a result
the H-vacancy cluster grows larger and larger. Finally, the H bubble
forms.